Repeated fault patterns in strip-like products are usually caused by faults in or erroneous operation of machine elements. For example, repeated faults will arise in the process of metal band rolling if there is a fault or anomaly in the working roll, causing it to imprint the final product at regular intervals. Correspondingly, in the process of paper manufacturing, faults in the rolls or incorrect operation of the calendar will cause repeated variations in paper quality. A roll generally refers to an approximately cylindrical machine element that is in contact with a strip-like product and rotates in the direction in which the product progresses. It must be noted that the roll may rotate at a speed different from the speed of the product.
In some cases one or more missing patterns within a regularly repeated pattern are considered a fault in a strip-like product. These include, for example, a watermark missing from watermarked paper or an imprint missing from printed adhesive paper.
Even if weak, such a cyclical fault is often more severe than an individual fault from the point of view of the metal roller or paper-maker, as well as the end client.
Automatic visual inspection of the surface of strip-like products is often based on CCD line scan cameras that take pictures of light reflected from the surface of the product at different angles. The web itself is moving, so an image can be created by reading a single line at a time at a high speed. The image signal from the CCD line camera is converted to a digital format and digitally processed in an image-processing unit. The end product of the process is a set of electronic images representing selected parts of the strip-like product.
The publication WO 02/078873 A1 presents a method for detecting a faulty roll in the manufacture of a strip-like product. The strip is inspected after the roll unit for any visual faults, and when one is detected the faulty roll is located by opening and closing roll pairs under timed control.
The publication U.S. Pat. No. 4,958,307 presents an apparatus for inspecting regular roll marks. The apparatus stores imprint data along the diameter of the roll into memory and compares the imprint data with reference data at each full revolution of the roll.